Carboxymethylation Of Cellulose From Kenaf (Hibiscus Cannabinus L.) Core For Hydrogel Production

Abstract

Kenaf is known as fast growing species which is planted mainly for bast fibre
which have been used as an alternative raw material for particleboard,
medium density fibreboard or pulp and paper industry. On the contrary, the
core of kenaf stem which is consist of straw-like and short-length fibres cannot
be used exclusively in particleboard and pulp paper industries due to low
strength and dimensional stability. Thus, and alternative usage has to be
researched. Since kenaf core have relatively high amount of holocellulose, low
lignin content and is highly absorbent, these properties make kenaf core a
potential material for producing carboxymethyl cellulose (CMC) in hydrogels.
This study comprised three main aspects: 1) Evaluation of the basic properties
of kenaf core fibres, 2) Carboxymethylation of the kenaf core and 3) Formation
of hydrogel from the carboxymethylated kenaf core.The evaluations of chemical and physical properties of kenaf core fibres were
conducted prior to the carboxymethylation process. The samples for fibre
morphology and chemical analyses of the kenaf core were prepared from
macerized and mechanically refined fibres, respectively. All the chemical
analyses were conducted using fibre of 40-mesh size (425 μm size) whilst for
fibre morphology (fibre length, fibre diameter, lumen diameter and cell wall
thickness) match stick kenaf core samples were used. The study indicates that
kenaf core comprises approximately 50.6% alpha-cellulose, 27.1%
hemicellulose and 20.5% of lignin. A high percentage of water absorption of
kenaf core (i.e., 50%) was recorded due to the presence of a large number of
pits along the longitudinal axis of the cell wall as shown by pictures from SEM.
The core fibres has low Runkel ratio of 0.36 which is responsible for high water
absorption owing to large lumen size and thin fibre wall suggesting good liquid
transmission.
Carboxymethylated kenaf core was successfully produced under
heterogeneous condition by reacting the core fibres with sodium
monochloroacetate as an etherifying agent and isopropanol as reaction
medium in the presence of sodium hydroxide as a swelling agent. Several
factors affecting the carboxymethylation process such as time, sodium
hydroxide concentrations and temperature were studied. The degree of
substitution (DS), reaction efficiency (RE) and weight percent gain (WPG)
were also determined. Among the three types of kenaf core used, crude kenaf core (CKC), extractive-free kenaf core (KCFE), and extractive- and lignin-free
kenaf core (KCFL), KCFL gives the highest DS value. The purity of the
holocellulose was found to be the crucial factor in obtaining high yield. The
optimum reaction condition for producing carboxymethylated kenaf core with
DS value of 0.87 was based on the following procedures: firstly, by extracting
the extractives and lignin from the kenaf core to produce extractive- and ligninfree
kenaf core (KCFL) and then reacting the KCFL with sodium
monochloroacetate in 30% sodium hydroxide at 55oC for 2 hours. From FTIR
spectroscopy analysis, the existence of peak at 1591 cm-1 on modified kenaf
core revealed that some of hydroxyl groups of cellulose kenaf fibre had been
effectively substituted with the carboxymethyl group. The SEM micrograph
observation also reconfirmed the substitution process, where the unmodified
kenaf core had rough features with the pits still empty. On the other hand,
smoother features were observed on the modified kenaf core fibre with the pits
has been filled presumably with carboxymethyl of sodium monochloroacetate.
Hydrogels were successfully produced using different concentrations of
carboxymethylated kenaf core of extractive- and lignin-free (CMKCFL) at
several irradiation doses. A dose of 70 kGy was found to be the optimum
irradiation dose for crosslinking of CMKCFL hydrogel with gel content of
37.1%. The optimum crosslinking condition was 50 kGy irradiation and 50%
concentration, producing an acceptably good properties CMKCFL hydrogel.
The swelling ratio of CMKCFL hydrogel was the highest in de-ionized water compared to alkaline (1.0 M NaOH) and acidic (1.0 M HCl) at all ranges of
irradiation doses except for 10 kGy. Increasing the level of concentration of
aqueous CMKCFL resulted in an increased in swelling. This result was
however opposite when the swelling test was conducted in acid and alkali
mediums. The overall results indicate that kenaf core fibre can be used as raw
material for carboxymethyl cellulose and hydrogel production and possess
satisfying other properties.